Hi, Kim.
I think that your message post "Have I done the right thing" indicates that you have some doubts. You certainly have been through quite a bit in the past few years. BUT, you have come through everything ok, and were able to put it all behind you. Certainly, it hasn't been easy (and I'm saying this without ANY concept of how difficult it HAS been for you), but you faced each problem head-on and continued until the end.
I can appreciate what you are thinking right now (again, without ANY concept of what you ARE thinking, but just trying to imagine myself in your position), just wanting a rest from medical problems and looking for just one time when everything is "normal" without a worry. Wouldn't it be wonderful if you could go through Christmas without doctor appointments, surgery, treatments, etc. and sit back and relax and enjoy a WELL-EARNED celebration??? You have more than earned this!!!
However, if you kow that you have something behind your eye, and you postpone your appointment until the new year, are you really going to be able to enjoy Christmas? Or will you, in the back of your mind, keep thinking about it and wondering what you will have to go through in the New Year?
You must be worried about this, and with everything you've come through, how could you not be worried? It might turn out to be something fairly minor that will go away on its own or perhaps need minimal treatment? Of course, you're worried about treament, surgery, etc. etc. etc.
For whatever reason, you've had to go through these difficult times and you HAVE, and done pretty well with them by the sounds of things. You can't stop now. You've come much to far to let another obstacle slow you down. If it is a minor something-or-other, then you'll feel tremendous relief in knowing that. If it is more serious (and I certainly hope it isn't), then you will at least know that you are dealing with it and going to make it better. Life has given you another test, you can't opt out of it. It will make everything you've done so far meaningless.
My advice to you is rebook the appointment and look after your health. It must have been a relief to you when you cancelled the appointment, knowing you wouldn't have to go through tests, appointments and treatments in the near future. However, I think the satisfaction you're getting in the short-term will give way to much more stress in the long-term.
Whatever you decide, I'm sure that you'll get support from everyone here. And it must be a difficult time for you. Perhaps life it giving you all of these problems right now, so that you can enjoy the rest of your life after this, and have dozens and dozens of peaceful, happy Christmases after this is all over.
Keep strong! My prayers are with you.
John